They're not, but it's still illegal to drive with headphones.
Deaf people still are unable to properly hear so are more unsafe when driving. They're legally allowed to do so because of deaf people lobbying legislators to force them to be allowed to do so.
Even assuming that’s true, if your hearing suddenly cuts out, and you decide to pull over in response, the chance you’ll encounter one of those objects in the few seconds it takes to pull over is rather low.
For this to be a real danger, either the driver would have to not notice that they suddenly couldn’t hear anything, or the faulty hearing aid would have to pass through some sounds but not others. Both of those scenarios seem pretty unlikely.
Meanwhile, from some quick googling, it seems to be common for completely deaf people to drive; apparently there are devices that can translate sirens and such into visual cues, but such devices are not universally used. To be fair, it’s a bit different if you’re not expecting it or practiced at it.
